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Physics 101: Lecture 27, Pg 1 Physics 101: Lecture 27 Thermodynamics l Today’s lecture will cover Textbook Chapter 15.1-15.6 l Check your grades in grade book Final

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Physics 101: Lecture 27, Pg 2 First Law of Thermodynamics Energy Conservation U = Q - W Heat flow into system Increase in internal energy of system Work done by system V P 1 2 3 V 1 V 2 P1P3P1P3 Equivalent ways of writing 1st Law: Q = U + W The change in internal energy of a system ( U) is equal to the heat flow into the system (Q) minus the work done by the system (W) 07

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Physics 101: Lecture 27, Pg 3 Signs Example l You are heating some soup in a pan on the stove. To keep it from burning, you also stir the soup. Apply the 1 st law of thermodynamics to the soup. What is the sign of (A=Positive B= Zero C=Negative) 1) Q 2) W 3) U Positive, heat flows into soup Zero, is close to correct Positive, Soup gets warmer 11

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Physics 101: Lecture 27, Pg 4 Work Done by a System ACT M M yy W = p V :For constant Pressure W > 0 if V > 0 expanding system does positive work W < 0 if V < 0 contracting system does negative work W = 0 if V = 0 system with constant volume does no work W = F d cos =P A d = P A y = P V 13 The work done by the gas as it contracts is A) PositiveB) ZeroC) Negative

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Physics 101: Lecture 27, Pg 5 Thermodynamic Systems and P-V Diagrams l ideal gas law: PV = nRT l for n fixed, P and V determine “state” of system è T = PV/nR è U = (3/2)nRT = (3/2)PV l Examples: è which point has highest T? » B» B è which point has lowest U? » C» C è to change the system from C to B, energy must be added to system V P A B C V 1 V 2 P1P3P1P3 17

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Physics 101: Lecture 27, Pg 6 First Law of Thermodynamics Isobaric Example V P 1 2 V 1 V 2 P 2 moles of monatomic ideal gas is taken from state 1 to state 2 at constant pressure p=1000 Pa, where V 1 =2m 3 and V 2 =3m 3. Find T 1, T 2, U, W, Q. (R=8.31 J/k mole) 1. PV 1 = nRT 1 T 1 = PV 1 /nR = 120K 2. PV 2 = nRT 2 T 2 = PV 2 /nR = 180K 3. U = (3/2) nR T = 1500 J U = (3/2) p V = 1500 J (has to be the same) 4. W = p V = +1000 J 5. Q = U + W = 1500 + 1000 = 2500 J 21

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Physics 101: Lecture 27, Pg 7 First Law of Thermodynamics Isochoric Example 2 moles of monatomic ideal gas is taken from state 1 to state 2 at constant volume V=2m 3, where T 1 =120K and T 2 =180K. Find Q. 1. Q = U + W 2. U = (3/2) nR T = 1500 J 3. W = P V = 0 J 4. Q = U + W = 1500 + 0 = 1500 J V P 2 1 V P2P1P2P1 24 requires less heat to raise T at const. volume than at const. pressure

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Physics 101: Lecture 27, Pg 8 Homework Problem: Thermo I V P 1 2 3 4 V P W = P V (>0) 1 2 3 4 V > 0V P W = P V = 0 1 2 3 4 V = 0 V P W = P V (<0) 1 2 3 4 V < 0 V W = P V = 0 1 2 3 4 P V = 0 V P 1 2 3 4 W tot > 0 W tot = ?? 27

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Physics 101: Lecture 27, Pg 9 WORK ACT If we go the opposite direction for the cycle (4,3,2,1) the net work done by the system will be A) PositiveB) Negative V P 1 2 3 4 If we go the other way then… 30

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Physics 101: Lecture 27, Pg 10 PV ACTs Shown in the picture below are the pressure versus volume graphs for two thermal processes, in each case moving a system from state A to state B along the straight line shown. In which case is the work done by the system the biggest? A. Case 1 B. Case 2 C. Same A B 4 2 39 V(m 3 ) Case 1 A B 4 2 39 V(m 3 ) P(atm) Case 2 P(atm) correct Net Work = area under P-V curve Area the same in both cases! 29

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Physics 101: Lecture 27, Pg 11 PV ACT 2 Shown in the picture below are the pressure versus volume graphs for two thermal processes, in each case moving a system from state A to state B along the straight line shown. In which case is the change in internal energy of the system the biggest? A. Case 1 B. Case 2 C. Same A B 4 2 39 V(m 3 ) Case 1 A B 4 2 39 V(m 3 ) P(atm) Case 2 P(atm) correct U = 3/2 (p f V f – p i V i ) Case 1: U = 3/2(4x9-2x3)=45 atm-m 3 Case 2: U = 3/2(2x9-4x3)= 9 atm-m 3 31

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Physics 101: Lecture 27, Pg 12 PV ACT3 Shown in the picture below are the pressure versus volume graphs for two thermal processes, in each case moving a system from state A to state B along the straight line shown. In which case is the heat added to the system the biggest? A. Case 1 B. Case 2 C. Same A B 4 2 39 V(m 3 ) Case 1 A B 4 2 39 V(m 3 ) P(atm) Case 2 P(atm) correct Q = U + W W is same for both U is larger for Case 1 Therefore, Q is larger for Case 1 34

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Physics 101: Lecture 27, Pg 13 Q = U + W Heat flow into system Increase in internal energy of system Work done by system V P 1 2 3 V 1 V 2 P1P3P1P3 Which part of cycle has largest change in internal energy, U ? 2 3 (since U = 3/2 pV) l Which part of cycle involves the least work W ? 3 1 (since W = p V) l What is change in internal energy for full cycle? U = 0 for closed cycle (since both p & V are back where they started) l What is net heat into system for full cycle (positive or negative)? U = 0 Q = W = area of triangle (>0) Some questions: First Law Questions 37

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Physics 101: Lecture 27, Pg 14 Special PV Cases l Constant Pressure (isobaric) l Constant Volume Constant Temp U = 0 l Adiabatic Q=0 V P W = P V (>0) 1 2 3 4 V > 0V P W = P V = 0 1 2 3 4 V = 0 42

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Physics 101: Lecture 27, Pg 15 Preflights 1-3 Consider a hypothetical device that takes 1000 J of heat from a hot reservoir at 300K, ejects 200 J of heat to a cold reservoir at 100K, and produces 800 J of work. Does this device violate the first law of thermodynamics ? 1. Yes 2. No “ w(800)= Qin (1000)-Qout (200) ” correct l W (800) = Q hot (1000) - Q cold (200) l Efficiency = W/Q hot = 800/1000 = 80% 80% efficient 20% efficient 25% efficient 45 orgo exam tomorrow...enough said. NOBODY BREAKS THE LAWS OF THERMODYNAMICS ON MY WATCH

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Physics 101: Lecture 27, Pg 16Reversible? l Most “physics” processes are reversible, you could play movie backwards and still looks fine. (drop ball vs throw ball up) l Exceptions: è Non-conservative forces (friction) è Heat Flow: »Heat never flows spontaneously from cold to hot 47

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Physics 101: Lecture 27, Pg 17Summary: è 1st Law of Thermodynamics: Energy Conservation Q = U + W Heat flow into system Increase in internal energy of system Work done by system V P l point on p-V plot completely specifies state of system (pV = nRT) l work done is area under curve l U depends only on T (U = 3nRT/2 = 3pV/2) for a complete cycle U=0 Q=W 50

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